On account of its structure corrugated cardboard in relation to its weight has a great strength in resisting loads. Hence, corrugated cardboard is an extremely good material, e.g. for different types of packings, such as cases or containers. Corrugated cardboard also is potentially suitable as material for construction elements, and to a certain extent it has already been used for this purpose.
In the construction as well as in the packing industry corrugated cardboard is a particularly attractive material not only for its favourable ratio between strength and weight but also for its heat insulating capacity and its low cost, especially in relation to different kinds of wooden material, e.g. plywood. The latter factor progressively is becoming of greater and greater importance due to the increasing shortage of wooden fibre materials on the world market.
Corrugated cardboard also contains fibres, but the content of fibres in corrugated cardboard is very small in relation to the strength of corrugated cardboard. While this strength is great in relation to the weight of corrugated cardboard, the absolute strength of corrugated cardboard, however, is not sufficient for many areas of use, e.g. as material for construction elements. Therefor, since a long time a desired end has been a possibility of increasing the absolute strength of corrugated cardboard without increasing the fibre content thereof. Experiments to this end have been carried out, in that liquid compositions have been introduced into the pipes of the corrugated cardboard and caused to become deposited on the walls of the pipes to be absorbed thereby and hardened in situ to form a stiffening layer increasing substantially the strength of the corrugated cardboard. The introduction of the compositions in question into the pipes of the corrugated cardboard so far has been made by immersing the corrugated cardboard sheets into baths of the compositions. This procedure has several disadvantages. Thus the method involves hardening, drying and handling problems. Also, with this procedure it is difficult to prevent the composition in question from coating also the exterior faces of the corrugated cardboard sheets, which would mar the appearance of the sheets and which might prevent the application of other materials to the sheets, e.g. dye or water-repellants or materials preventing water from penetrating into the corrugated cardboard to weaken or destroy it. Also the method of causing a stiffening composition to be deposited on the interior walls defining the pipes by immersing the corrugated cardboard sheets into a bath of such a composition is a batch process. Of course, replacing such a batch process by a continuous process is desirable since a continuous process always means considerable savings in time and work and hence substantial cost savings, and obviously offers extremely favourable conditions for mass production as compared to batch processes. With the latter processes it is also difficult to closely control the feed of the compositions in question to allow the pipe walls to absorb neither more nor less than what is required to obtain the desired stiffening.